Researchers say most of the water vanishing from the Aral Sea and the Great Salt Lake is now in the oceans of this increasingly parched world.
Climate change has begun to dry out the heart of almost every continent. This parched world’s landlocked basins – they make up a fifth of the Earth’s surface – have lost at least 100 billion tonnes of water every year since the century began. And US researchers now know where that water has gone.
Groundwater, lake and inland sea evaporation from inland Australia, the US West, the Chilean deserts, Saharan Africa, the Middle East and central Asia is now in the oceans, to account for 4mm, or at least 8%, of global sea level rise so far.
In effect, many of the world’s arid zones are becoming progressively more arid, according to a new study in the journal Nature Geoscience.
“Human activities such as groundwater depletion are significantly accelerating this drying”
Researchers used 14 years of observation by a set of orbiting satellites – known as GRACE, for Gravity Recovery and Climate Experiment – to observe the steady desiccation of regions that geographers know as endorheic basins. These are inland regions into which mountain streams, subterranean flows and sluggish rivers drain: among them the Caspian and the Aral Seas in Eurasia, and the Great Salt Lake in the US.
They are very different from the world’s great exorheic basins, better known as the Nile, the Amazon, the Mississippi and the Yangtze, all of which flow into the sea.
People in exorheic basins know their water supply will always be replenished. People farming or grazing cattle in the endorheic basins can now see their most vital resource slowly vanishing.
Evidence mounts
“Over the past few decades, we have seen increasing evidence of perturbations to the endorheic water balance,” said Jida Wang, a geographer at Kansas State University, who led the study.
“This includes, for example, the desiccating Aral Sea, the depleting Arabian aquifer and the retreating Eurasian glaciers. This evidence motivated us to ask: Is the total water storage across the global endorheic system, about one-fifth of the continental surface, undergoing a net decline?”
The GRACE satellites have already answered a series of huge questions about the world’s traffic in ice and water: they have “weighed” the loss of ice in the Antarctic, and put a total to the impact of devastating floods in Australia in 2011.
Speed of disappearance
And the remote sensing instruments now deliver a measure of the rate at which endorheic water is disappearing. Not only does it account for nearly one tenth of sea level rise so far, it adds up to nearly half the loss of water from retreating mountain glaciers in the densely occupied countries – that is, excluding Greenland and Antarctica – and it matches the entire extraction of groundwater, everywhere in the world, for irrigation and to nourish towns and cities in the drier regions.
The parching of the inland basins is uneven – some report more rainfall – but around 75% have been steadily getting drier. “The water losses from the world’s endorheic basins are yet another example of how climate change is further drying the already dry arid and semi-arid regions of the globe,” said Jay Famiglietti, one of the co-authors, who directs the Global Institute of Water Security, at the University of Saskatchewan in Canada.
“Meanwhile, human activities such as groundwater depletion are significantly accelerating this drying.”
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